Liquid crystal display devices utilize optical anisotropy and dielectric anisotropy of liquid crystalline compounds. As driving mode of the liquid crystal display devices, active matrix driving mode using a TFT has attracted most public attention since it has a high display capability.
While various properties are required of the liquid crystalline compounds used for the driving mode described above, the followings are generally considered to be necessary:
1) Liquid crystalline compounds have a wide temperature range of nematic phase to actualize a wide range of a driving temperature. Alternatively, the compounds do not reduce the temperature range of a nematic phase or they hardly cause phase separation such as formatin of crystals at a low temperature region when they are added in liquid crystal compositions.
2) Liquid crystalline compounds have a low viscosity to actualize a high response speed.
3) Liquid crystalline compounds have a high optical anisotropy.
In order to realize liquid crystal compositions having such properties as described above, a means has been adopted wherein liquid crystalline compounds having a low viscosity, very wide temperature range of a nematic phase, and particularly a high clearing point are used as base liquid crystal, and a proper amount of other liquid crystalline compounds having a distinctive optical anisotropy are mixed therewith to obtain desired properties.
In order to obtain liquid crystal compositions which can be driven at a wide temperature range, liquid crystalline compounds having a driving temperature range as wide as possible, that is, a S-N point or melting point as low as possible, a clearing point as high as possible, and a temperature range of a nematic phase as wide as possible are necessary.
Further, liquid crystal compositions are generally composed of a mixture of several or thirty-odd liquid crystalline compounds to exhibit properties required for particular display devices. Accordingly, liquid crystalline compounds are required to have a good miscibility with other liquid crystalline compounds. Since environment of their use extends over a wide area lately, good miscibility at low temperatures is especially required.
Namely, liquid crystal compositions are necessary to have a nematic phase especially at low temperatures for their use in a wide temperature range, and liquid crystal compositions which do not form crystals or do not exhibit a smectic phase are required. Accordingly, it is extremely important that liquid crystalline compounds to be used have a high miscibility at low temperatures with other liquid crystalline compounds.
Various tetracyclic compounds or four ring compounds have already been disclosed, for example, in Laid-open Japanese Patent Publication Nos. Hei 4-312,540 and Hei 4-356,432, aiming at the purposes of simultaneously satisfying the wide temperature range of a nematic phase, good miscibility, and large optical anisotropy described above. Besides, compounds expressed by the general formula of ##STR2##
are disclosed in Laid-open Japanese Patent Publication No. Sho 58-203,922 (JP '922) and the compounds expressed by the general formula of ##STR3##
are disclosed in Laid-open Japanese Patent Publication No. Hei 2-237,949 (JP '949), respectively.
Whereas the compounds of either publication exhibit a high optical anisotropy, ones of JP '922 are very poor in the miscibility and ones of JP '949 have a narrow temperature range of a nematic phase. In Mol. Cryst. Liq. Cryst., 1985, Vol. 123, pages 1-13, GB-A-2240 778, WO-A-91 08184, and GB-A-2 134 110, various tetracyclic compounds have been disclosed. However, these compounds are not satisfactory in their temperature range of nematic phase, clearing point, and miscibility with other liquid crystalline compounds. Accordingly, new liquid crystalline compounds having more excellent properties were long-awaited.
Liquid crystalline compounds used for liquid crystal compositions must be stable against outside environmental factors such as moisture, air, heat, and light. Particularly, the liquid crystal compositions designed for liquid crystal displays of active matrix mode which comprise integrated non-linear devices for switching individual image segments must have an extremely high specific resistance (a high voltage holding ratio) and good UV stability (UV resistance).
Liquid crystal display devices of active matrix mode are suitable for TV sets, computers, and instruments used in automobiles or airplanes all of which display a high degree of information. However, when liquid crystalline compounds or liquid crystal compositions which do not have an extremely high specific resistance (a high voltage holding ratio) and good UV stability are used, contrast decreases with decreasing of electric resistance within a liquid crystal panel, which causes a problem of "phenomenon of image sticking". Particularly, when the liquid crystal display devices are driven at low voltages, electric resistance of the liquid crystal compositions is an extremely important factor which controls utility life of the devices. Thus, an extremely high specific resistance (high voltage holding ratio) and good UV stability are very important properties required of liquid crystal compositions to be used.
In order to provide the liquid crystal compositions having excellent properties, new liquid crystalline compounds having a wider temperature range of a nematic phase, high miscibility with other liquid crystalline compounds at low temperatures, high chemical stability, and high optical anisotropy simultaneously were long-awaited.